It's best to cover the ball with a coordinating color of fondant just in case the ball peeks out between flowers. It's also a lot easier to stick the flowers onto fondant than onto styrofoam. Brush a little water or piping gel mixed with water on the ball to help the fondant stick.

Insert a skewer into the bottom of the ball and anchor it in another piece of styrofoam so it's easier to work with. Eventually, the skewer will be inserted into the cake. It'll be a lot easier to work with if you trim a little off the bottom of the ball first.